As is known in the art, a modulator-demodulator (modem) is an electronic device that modulates transmitted signals and demodulates received signals. The modem generally provides an interface between digital devices and an analog communications system to thus make possible analog transmission of digital information between two terminals or stations. Such transmissions may be over a transmission link such as a telephone line, cellular communication link, satellite link, and cable TV, each of which being generally band-limited. That is, the information may be transmitted across the transmission link only over a predetermined range of frequencies having a maximum bit error rate.
As is also known, a modem is used to provide wireless transmission between transmitting and receiving stations. Such wireless communication can be employed in a variety of applications: VHF, IS-54 (cellular), IS-95 (cellular), SPADE (satellite), GSM (cellular), HDTV, and SAT-TV, each using one of the following linear modulation techniques: QAM; QPSK; Pi/4 DPSK; GMSK. As with the transmission line application, the bandwidth of each is limited within an acceptable bit error rate.
While most modems are capable of providing compensation for Guassian noise, impulse noise is not well managed. Most modems also require higher powered amplification means since, in all cases, amplitude distortion is unacceptable. In broadcast environments, it is well understood that FM transmission provides superior impulse noise handling. Also in FM transmission schemes, the transmitted signal may be amplified with almost 100% efficiency since the information carrying portion of the signal is identified by the zero-crossings of the signal. Thus, amplitude distortion is ignored.
Despite these apparent advantages, only a few isolated examples exist of nonlinear frequency modulation used within modems. Frequency shift keying, providing a 300 bits per second data rate, essentially utilized two frequencies, each for representing a separate data bit. The next major development in modems was the use of phase modulation, beginning with two phase modulation, then four, then eight. A combination of amplitude and phase modulation was later developed, also referred to as quadrature amplitude modulation or QAM.
A subsequent development was Guassian minimum shift keying, or GMSK. At first glance, such coding resembles four phase modulation, though in order to avoid amplitude modulation, a special low pass filter referred to as a Guassian filter was applied to the data going into the phase modulator. Since some have regarded the Guassian filter as akin to an integrator, an argument could be made that such a modulation scheme results in frequency modulation, since GMSK can be demodulated with an FM discriminator. Yet, GMSK applies linear functions of the data to in-phase and quadrature carriers to produce a linear modulation; whereas, true FM is mathematically equivalent to the application of trigonometric functions of the filtered signal to an in-phase and quadrature carrier. FM is a non-linear modulation, which was perceived as inefficient for data transmission since the transmitted frequency spectrum is not a simple translation of the baseband spectrum as in AM modems. A further perceived problem with FM for a modem is that it only accepts real signals into its voltage controlled oscillator. That is, in the equivalent in-phase and quadrature carrier method for FM, both paths send the same data resulting in a double sideband spectrum of a single carrier, which was regarded as redundant and therefore inefficient compared to the double-sideband spectra of the dual in-phase and quadrature carriers of QAM modems. Also, single-sideband (SSB) transmission was considered undesirable for modems because there is no simple way to efficiently demodulate an AM-SSB signal without a carrier reference. FM was dismissed as an analog technique totally unrelated to data transfer except with regard to FSK.
In an ordinary modem, binary data is normally passed through a baseband raised-cosine filter which limits the bandwidth of the baseband signal so that when one multiplies the baseband signal by a carrier, control over the passband signal bandwidth is provided without intersymbol interference. The output of an ordinary modem includes signals having discrete phases such that data included therein can be identified by discerning the phase of each bit. For instance, whenever a signal has a +90.degree. phase shift it is interpreted as 0 and when the phase shift is -90.degree. it represents 1, etc. Thus, in an ordinary modem employing a carrier, the phase and/or amplitude of the carrier signal are determined by the current symbol being transmitted. The carrier assumes only selected values of phase and amplitude for most of the duration of each symbol and graphical plots of all the selected phase-amplitude pairs are called the constellation points of the modem. ordinary modems require that there be distinct points in the constellation for each possible value of the transmitted symbol. Furthermore, bit errors occur in the receiver if the points are mis-assigned due to intersymbol interference or due to noise on the link.
In many applications the computational requirements of the modem introduce a delay which is detrimental to the operation of the system. For example, digital voice transmission and multiple access networks are sensitive to delay in the modem. Furthermore, the rate at which a modem may transmit and receive data per unit of bandwidth is called the modem bandwidth efficiency. In the discipline of Digital Information Theory this efficiency is known to be maximized when the transmitted signal has the maximum entropy or randomness. The maximum entropy transmission is band-limited Guassian noise, and among other properties Guassian noise will not dwell at a distinct phase-amplitude pair as in a constellation. Thus it is desirable to provide a modem having a passband carrier which minimizes the internal processing time and also maximizes the bandwidth efficiency without sacrificing bit error rate.